High-frequency amplifier circuits



May 13, 1930. a BRADBURY HIGH FREQUENCY AMPLIFIER CIRCUITS Filed March 2. 1928 [/ecfrosfafica/ly Coup/ed Inventor B We Bradbury y His Attorney Patented May 13, 1930 UNITED STATES PATENT OFFICE BURKE BRADBUBY, OF SGHENECTADY, NEW YORK, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK HIGH-FREQUENCY AMPLIFIER CIRCUITS Application filed March 2,

My present invention relates to circuits employed for the purpose of amplifying high frequency currents, and particularly currents of the frequencies employed for radio signaling purposes.

One of the objects of my invention is to provide an amplifying system which is capable of amplifying efliciently, high frequency voltages over a wide range of frequenci'es such, for example, as the range of frequencies employed'for radio broadcasting.

A more specific object of my invention is to provide a circuit arrangement which will be capable of producing a substantially uniform degree of amplification over the complete range of frequencies for which it is intended to be employed. a

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation will best be understood by reference to the following description taken in connection with the accompanying drawing in which I have shown diagrammatically a circuit organiza tion in which my invention is embodied.

, I have indicated in the drawing a system for the reception, amplification and detection of radio signals, comprising a plurality of vacuumtubes 1 to 5 inclusive, which are supplied with circuit connections whereby radio signals received upon the antenna 6 may be amplified and detected and supplied in suitable intensity to the output terminals 7 of the system.

Tubes 1 and 2 are of the type now known as four-element screen-grid tubes. The signals received upon the antenna 6 are impressed upon the control grid 8 of tube 1 this grid being directly connected to the antenna system. The output circuit of tube 1 includes the primary 9 of a coupling trans former, the secondary 10 of which forms a part of a tuned link circuit connecting the output circuit of tube 1 with the input circu-it of tube 2. This transformer may have a one to one ratio and be closely coupled so as to alford an efiicient energy transfer to the linkcircuit. The impedance of the output 1928. Serial N'o. 258,558.

circuit of tube 1 is so high that a close coupling in this case Will not materially affect the selectivity afiorded by the tuned link circuit. A magnetic coupling is provided between this tuned link circuit and the input circuit of tube 2 by meansof a coupling trans- I former having a primary winding or coupling coil 11 and a secondary winding 12. The link circuit and the input circuit of tube 2 may be tuned in the usual manner by means of variable condensers 13 and 1% operated simultaneously by a single control means. The electromagnetic coupling furnished by the coils 11 and 12 is preferably very loose. I have found, for example, that the primary winding or coil 11 may consist of only one and one-half turns wound close to the secondary winding 12 consisting of sixty-four turns, about two inches in diameter. The two coupling transformers are so arranged that there is no magnetic coupling between them. This may be accomplished by placing them at right angles to each other or by other known methods.

These circuits, however, should be so arranged that there is a certain amount of electrostatic coupling between the tuned link circuit and the input circuit of tube 2 as indicated in the drawing. This may be the natural electrostatic coupling existing between the two circuits. I have found that by suitably proportioning the relation between the electromagnetic and electrostatic couplings between the two circuits the combined action of the two couplings will, with a given strength of signal, produce a substantially uniform voltage transfer from the output circuit of tube 1 to the input circuit of tube 2 over the entire operating range of the system. This may be accomplished by varying the number of turns in primary coil 11 and its magnetic relation to secondary 12 and by varying the physical relation between tuned circuit 10-13 and tuned circuit 12-14 to vary the electrostatic coupling between them.

The electrostatic coupling should beso arranged that the potential supplied thereby to the input circuit is substantially 180 out of phase with the potential fed through the magnetic coupling. The electrostatic cou pling must be so proportioned that it will never be of greater magnitude than the magnetic couplin If the two were of the same magnitude at a particular frequency no voltage transfer would result and the arrangement would be inoperative at that frequency. In practice therefore the couplings may be adjusted at the highest frequency at which the circuit is intended to operate in such a way as to get maximum sensitivity without too much effective coupling. At lower frequencies the effects of the two couplings will Vary but the effective coupling will. always be the difference between the two, and this may bemade to be substantially constant.

lVhile the desired electrostatic coupling may be obtained by suitable relation of the various elements of the circuit as above described it may in some cases be desirable to provide electrostatic shielding means between the circuits and provide a desired equivalent electrostatic or condenser coupling of suitable value by other means than that fur ished by the physical relation between the circuits. I have indicated the magnetic coupling as being made at the low potential ends of the coupling coils. This is preferable because of the fact that it reduces the capacity coupling between the windings, which would otherwise be of too great a magnitude.

The output circuit of tube 2 is coupled by means of a coupling transformer 15 to the input circuit of tube 3, which is of the usual three electrode type, and is connected to serve as a detector. The input circuit of this tube is tuned by means of a tuning condenser 16 and the tuning condensers 13, 14 and 16 may all be connected on the same shaft to provide unicontrol. The output circuit of tube 3 includes a tickler coil 17 to provide regenera tion and the resulting increased amplification. This tickler coil 17, instead of being made adjustable, is fined, but is so arranged to give a substantially uniform degree 0' regenerative amplification over the entire operating range of the system. This effect is accomplished by shunting a portion of the coil 17 by a condenser 18 and a resistance 19.-

The condenser 18 and the portion of coil 17, which it shunts, are so selected and proportioned that the parallel circuit formed thereby will tend to resonate at a frequency somewhat lower than the lowest frequency of the range over which the circuit is intended to Operate. The fixed tickler arrangement, as is well known, is most effective at the higher frequencies and its effectiveness falls off materially at the lower frequencies. As the parallel circuit described approaches resonance it increases the amount of feedback provided by the tickler and therefore increases the regenerative amplification at the lower frequencies. If the tuning of this parallel circuit is very sharp, however, this will only be of. benefit over a short range of the lower frequencies to be amplified. By the insertion of resistance 19 which may be of fairly high value the resonance of this parallel circuit will be broadened so that the advantage gained thereby will be spread out over a much wider ran e of the frequencies to be amplified. I have found that by suitably proportioning of the circuit the amount of regenerative amplification obtained may be made to be substantially constant over quite a wide range of frequencies such, for example, as the range of frequencies employed for broadcasting. The output from the detector 3 is supplied to the audio amplifier tube a and the output from tube 4 is supplied to a power amplifier tube 5 in the usual manner which requires no further detailed description. The various potentials required for operating the system may be supplied in any suitable manner to terminals 20 to 27 inclusive.

I have shown in the drawing a series capacity 30 and a series inductance 31 connected between antenna 6 and ground, the control grid of the coupling tube 1 being connected to the point 32 between the capacity 30 and inductance 31. The antenna circuit is preferably so proportioned that it is not resonant at any point over the operating range of frequency of the circuit. By proportioning the antenna circuit so that it approaches resonance at the lower frequencies to be amplified, I have found that with the arrangement illustrated it is possible to obtain a substantially uniform signaling voltage on the grid of the first tube for uniform signal strength over the operating range of the set. At the lower fre quencies the voltage on the grid is increased somewhat by reason of the fact that the an tenna tuning which is naturally broad is approaching resonance. This effect of course decreases as the frequency increases. On the other hand, the natural reactive drop across inductance 31 increases as the frequency increases. By proper choice of the constants of the circuit one of these variations may be made to substantially ofiset the other and provide substantially uniform results over the entire operating range. A series condenser 30 is employed of fairly small capacity in order that the apparatus may be used with a wide variety of antennae, and in order that the performance of the set as a whole will not depend greatly upon the capacity of the particular antenna with which it is employed. That is, the capacity 30, being small will in all cases limit the effective capacity of the circuit, although the capacity of the antenna in series with it may vary over quite a wide range.

WVhat I claim as new and desire to secure by Letters Patent of the United States, is,

1. The combination in a radio receiving sysetm adapted to receive signals within a Lei specified range of frequencies, of a four-element screen-grid electron discharge device having a tunable link circuit closely coupled to its output circuit, and a second four-element screen-grid electron discharge device having a tunable input circuit, a portion of said tunable link circuit being coupled to the tunable input circuit of the second device by an electromagnetic coupling, said circuits being arranged to have a certain electrostatic coupling between them, and said couplings being so proportioned that the voltage trans fer from the output circuit of the first device to the input circuit of the second device will be substantially uniform through the specified range of frequencies.

2. The combination in a radio receiving system, of a pair of four-element screen-grid electron discharge devices and means for transferring energy from the output circuit of one of said devices to a tunable input cir cuit of the second device over a specified range of frequencies, said means comprising a link circuit which is tunable over the specified range and which includes a secondary winding closely coupled magnetically to the output circuit of the first device and a coupling coil loosely coupled magnetically to the input circuit of the second device, said link circuit being so arranged with respect to said input circuit that it is electrostatically coupled therewith, said magnetic and electrostatic couplings with the input circuit of the second device being so proportioned that the voltage transfer between the two devices will be substantially uniform through the specified range of frequencies.

3. The combination in a radio receiving system adapted to receive signals within a specified range of frequencies, of a four-element screen-grid electron discharge device having a tunable link circuit closely coupled to its output circuit, and a second four-element screen-grid electron discharge device having a tunable input circuit, a portion of said tunable link circuit being coupled to the tunable input circuit of the second device by an electromagnetic coupling, said link circuit being arranged in such relation to said input circuit that it provides an electrostatic coupling therewith, said couplings being so arranged that the voltage supplied to the input circuit by one of the couplings is substantially 180 out of phase with the potential supplied by the other coupling.

4. The combination in a radio receiving system adapted to receive signals within a specified range of frequencies, of a fourelement screengrid electron discharge device having a tunable link circuit closely coupled to its output circuit, and a second four-element screen-grid electron discharge device having a tunable input circuit, said tunable link circuit being arranged with respect to the tunable input circuit of the second device in such a manner that it is electrostatically coupled therewith, and including a coupling coil electromagnetically coupled with said tunable input circuit, and the two couplings being so proportioned that the difference between the voltages supplied by the two couplings will be substantially uniform throughout the specific range of frequencies.

5. The combination in a radio receiving system adapted to receive signals within a specified range of frequencies, of a four-element screen-grid electron discharge device, a tunable link circuit including a secondary winding closely coupled to the output circuit of said device, and a second four-element screengrid electron discharge device having a. tunable input circuit, a coupling coil in said tunable link circuit, said coil being electromagnetically coupled to the tunable input circuit of the second device, said tunable link circuit and said tunable input circuit being arranged to provide an electrostatic coupling between them, the voltage supplied by the electrostatic coupling being of smaller value than and substantially 180 out of phase with the voltage supplied by the electromagnetic coupling, and the two couplings being so proportioned that the difference between the voltages supplied by the two couplings will be substantially uniform throughout the specified range of frequencies.

6. The combination in a vacuum tube radio receiving system adapted for operation over a specified range of frequencies, of a fixed antenna system which is not resonant to any frequency within the operating range of the system, said antenna system being arranged and proportioned to impress upon the control grid of a vacuum tube substantially equal voltages for equal signal strengths over the entire operating range of the system, a selective coupling system for transferring voltage to the tuned input circuit of a vacuum tube amplifier, said coupling system including a screen grid vacuum tube having a control grid connected with said antenna system, an output transformer for said tube having a tunable secondary and a coupling coil in series therewith, said coupling system being also arranged to transfer substantially equal voltages for equal signal strengths over the entire operating range of the system, and a regenerative detector having a fixed feed-back coupling system arranged to give substantially uniform amplification over the entire operating range of the system.

In witness whereof, I have hereunto set my hand this 1st day of March, 1928.

BURKE BRADBURY. 

